"Eritrea resumed its activity in (the Intergovernmental Authority on Development in Eastern Africa) IGAD and took its seat at the 14th Ordinary Summit in Djibouti," Information Minister Yemane Meskel wrote on Twitter late on Monday after the summit.
He did not say what had prompted the decision but said Eritrea wanted to join other IGAD members and help advance peace and stability in the region.
The Asmara government quit IGAD in 2007 to protest Ethiopian troops' entry into Somalia to force out its Islamist rulers.
Eritrea, which has been ruled by Isaias Afwerki since gaining independence from Ethiopia in 1993, fought a bloody border war with Ethiopia from 1998 to 2000, and has repeatedly fallen out with its neighbors.
However, it has taken steps in recent years to repair regional ties.
In 2018, it signed a peace deal with Ethiopia that formally ended the state of war that had existed since the border conflict, restored diplomatic ties with Somalia and normalised relations with Djibouti.
Eritrea is also under U.S. and E.U. sanctions for alleged human rights abuses.
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